Although there are characteristics whose inheritance adheres strictly to Mendel's predictions, more often than not, that is not the case. Many different mechanisms can alter the phenotypic expression of an inherited genotype, complicating attempts to predict offspring phenotypes.

One such case is called incomplete penetrance. If a phenotype is determined by a dominant allele, it is expected that all individuals bearing the dominant allele will show the phenotype. Sometimes, the phenotype is not expressed in some individuals, or the phenotype seems to skip generations, only to reappear later. If all individuals in a population express the phenotype, its allele is said to have complete penetrance. The degree of penetrance is described on the level of population--in what percentage of individuals having the dominant allele the phenotype will be expressed. An example of a gene with incomplete penetrance is that causing polydactyly (extra...